(a) Technical Field
The present invention relates to a metal-sulfur electrode for a lithium-sulfur battery and a method for preparing the same. More particularly, it relates to a metal-sulfur electrode for a lithium-sulfur battery prepared by coating a slurry mixture including sulfur, a conductive material and a binder as an electrode active material on a metal electrode and drying the same while applying an electric field so that the conductive material is aligned adequately so as to provide maximized efficiency during repetitive charging and discharging when used in the anode of the lithium-sulfur battery.
(b) Background Art
Metal-sulfur electrodes are typically used as anodes in a lithium-sulfur battery. Lithium-sulfur batteries are also known as high-performance lithium batteries in the vehicle industry. In the metal sulfur electrode, a conductive material is typically used since sulfur is an insulator and lacks electrical conductivity. The conductive material included in the metal-sulfur electrode provides a conduction path for lithium ions to travel to sulfur as well as an electron conduction path between a metal (e.g., aluminum) electrode and the sulfur. To apply the lithium-sulfur battery to electric vehicles with mileage comparable to that of gasoline-fueled vehicles, the energy density should be between 300-500 Wh/kg. To achieve this, the active material is formed in a relatively thick manner on the aluminum electrode to increase the amount of sulfur per unit area. As the amount of sulfur increases the amount of conductive material must increase as well. However, increasing the amount of conductive material may lead to reduced conductivity of the electrode as charging and discharging are repeated over time. To overcome this problem, an electrode having pores allowing for easy transportation of polysulfide is necessary.
The lithium-sulfur batteries have a theoretical energy density of 2,600 Wh/kg, which is much higher than 570 Wh/kg of the existing lithium-ion batteries. However, in order to increase the energy density of a battery pack for use in a vehicle, the proportion of sulfur should be maximized in an active material consisting of sulfur, a conductive material and a binder. Furthermore, the current lithium sulfur batteries also suffer from decreased capacity and lifetime after repeated charging and discharging.
The existing metal-sulfur electrode for a lithium-sulfur battery requires a large amount of conductive material to provide improved conductivity since the conductive material is not aligned in a coating layer of sulfur, the conductive material [e.g., carbon nanotube (CNT)] and a binder on a metal electrode, e.g., an aluminum electrode, as shown in FIG. 1. This causes decreased energy density. Furthermore, the electrode is ineffective since the sulfur is not uniformly dispersed in the conductive material during recharging following discharging. In addition, it has a short lifetime because of poor durability.
In this regard, Japanese Patent Application Publication No. 2005-251586 discloses a lithium secondary battery which includes a positive electrode current collector in which sulfur, a conductive material and a binder are included. However, the above-described problem associated with the use of the conductive material is not solved in this method.
Furthermore, Japanese Patent Application Publication No. 2006-143496 discloses formation of carbon nanotubes on a substrate and orientation of the carbon nanotubes in a desired direction by applying an electric field. However, it merely relates to a technique of aligning carbon nanotubes in a desired direction using an electric field without regard to sulfur. Hence, it does not solve the above-described problems associated with the metal-sulfur electrode.
Even further, Japanese Patent Application Publication No. 2011-23276 discloses a current collector which includes a resin layer having conductivity, the resin layer includes an electric field-responsive polymer material expanding or contracting reversibly in response to application of an electric field. However, the conductive material is not related to the metal-sulfur electrode at all and sulfur is not used in this patent.
Korean Patent Application Publication No. 2007-17062 discloses a method of forming a direct-current electric field between electrodes to induce formation of conductive materials and active materials. However, this patent is also not related to the metal-sulfur electrode.
That is to say, the existing techniques fail to solve the problem of providing effectively maintained conductivity in an active material which includes sulfur, a conductive material and a binder in a metal-sulfur electrode for a lithium-sulfur battery in order to increase energy density.